Rubber may be molded onto a metal insert to form various types of parts, including powertrain mounts. The metal insert is placed into a mold cavity shaped like the finished molded part, and the mold is closed. Rubber is injected into the mold, surrounding all or a portion of the metal insert and conforming to the shape of the mold cavity.
When multiple parts are molded simultaneously, a three-plate multi-cavity mold may be used. The top plate typically contains a single opening through which a stream of uncured rubber is injected into the mold. The bottom plate contains multiple cavities shaped like the finished molded part, each cavity having one or more openings through which the rubber flows. The middle plate, often called a “runner” plate, separates the stream of rubber that is injected through the top plate and directs it into the multiple cavities in the bottom plate. Because the rubber is viscous, high pressures (e.g., 200 MPa) are required to force the rubber through the single opening in the top plate, across the runner plate, and into each of the mold cavities below.
Once the parts are molded, they are heated to harden or cure the rubber. A typical temperature for curing rubber is 320° F., and a typical time is 10 to 15 minutes. The mold remains closed until the rubber is cured, exposing not only the mold cavity, but also the runner plate to the curing temperatures. Because rubber cannot be melted and reused once it has been cured, the hardened rubber in the channels of the runner plate must be removed after each molding process and thrown away. For this reason, runner plates are typically designed to provide maximum direction of the injected rubber with minimum waste.
Metal inserts are commonly shaped by stamping, resulting in slight variations in size and shape. Manufacturing tolerances for stamped metal inserts can be roughly 0.5 mm. The molds must be made slightly larger than the inserts to allow for the manufacturing tolerances. Because the rubber is injected at high pressure, it enters all available cavities, flowing into the spaces provided to accommodate the manufacturing tolerances of the metal inserts and coating even those portions of the inserts that must be free of rubber. In the case of a powertrain mount, for example, the section of the part that mounts to the engine must be free of rubber. The unwanted, excess rubber is called “flashing.”
A number of processes have been used to avoid flashing or to remove it after molding. A typical means of avoiding flashing is to place a bite ring onto the metal insert at a point at which the rubber is to stop. The bite ring cuts into the metal, preventing the rubber from flowing past the bite ring and onto the portion of the insert that is intended to be rubber-free. Bite rings have a number of disadvantages. Because they actually “bite” into the metal, they may leave marks on the metal or cut into any coating that has been applied to the metal prior to molding. Bite rings also wear out quickly and must be replaced frequently. If a bite ring is not replaced at the first sign of wear, it may leak. In addition, a bite ring may be used only in a plane perpendicular to the mold operating plane. This prevents its use on any portion of a metal insert that is positioned vertically in the mold cavity.
If a bite ring or another means of preventing flashing is not used, the flashing must be removed after the part has been molded. A typical way of removing the flashing is by scratch brushing the part. Again, this can leave marks on the metal or remove a coating that has been applied to the metal before molding. If the metal insert is coated after the molding step, rather than before, extra care must be taken to avoid applying the coating to the rubber in addition to the metal insert.
It would be desirable, therefore, to provide a method of forming a molded powertrain mount that overcomes the aforementioned and other disadvantages.